1. Field of the Invention
This invention relates to wheel constructions, and more particularly, wheels used for casters in furniture, carts, and other movable objects.
2. Description of Related Art
Typically, caster designs tend to fall into one of two groups: the ‘single wheel’ and the ‘twin wheel’. The single wheel caster (FIG. 1 is representative) consists of one wheel with an axle through the center which is attached to a clevis-like bracket that supports the axle at opposed ends. The bracket is joined to a bearing assembly that allows the caster to pivot about a generally vertical axis while also permitting rotation of the wheel about a generally horizontal axis, thereby allowing the caster to roll in any direction. 
The other main type of caster design is the ‘two wheel’ caster (as represented in FIGS. 2 and 3), in which two wheels share a common axle, but are free to rotate separately. The axle is affixed to a central caster body, from which a vertically extending pivot shaft extends to the movable object. In this way, each wheel is free to rotate in opposite directions, facilitating a change in direction of the movable object. Also, the wheels may turn in the same direction for straight-line movement of the object.
The main problem of the single wheel caster is its inability to turn easily about the pivot. When changing direction, the wheel is required to pivot about the vertical axis of the bracket bearing assembly. Differing portions of the contact area of the wheel tread travel arcs of differing radii, requiring differing linear velocities, thus necessitating that some of the contact area is in rolling contact while other portions are sliding and dragging. The result is wear of the wheel, and greater rolling resistance, particularly at low speeds and tight turns. A narrower wheel minimizes this problem, but results in greater pressure (force per unit area) applied to the floor surface.
The two wheel caster offered an improvement over the single wheel in two important regards. The ability of the wheels to rotate at differing rates or in opposite directions at the same time greatly enhances the ability to turn about the vertical pivot axis, making a change in overall direction of the object much smoother. Also, the separation of the two wheels establishes a wide base for stability. One problem germane to this caster type is the method of attachment of the wheel to the axle. Unlike the single wheel with the clevis bracket supporting the outer ends of the axle, the two wheel design typically attaches each wheel to  its respective end of the axle, the vertical pivot shaft being disposed in a plane between the two wheels. The axle is supported medially, and the wheels and the ends of the shaft are relatively unsupported, unprotected, and vulnerable to impact, which can result in collision-damaged wheels or a bent axle. This attachment configuration also creates some inherent stability inefficiencies. By not allowing the axle to extend through the wheel, the loading of the wheel on the axle is not symmetrical. That is, the inside of the wheel is fully loading the axle while the outside is not loading the axle at all. Also, the limited space remaining in the device for the wheel thickness results in thin wheels, which directly result in narrow annular (limited) bearing surfaces of the wheel on the axle. The two wheel design is typically limited to use in office furniture, where more casters are used (i.e., 5 or 6 casters per chair) and the loads on each caster thus are lighter and the objects are not often moved.
The prior art is deficient in providing a wheel that provides the advantages of independently rotating tread portions while avoiding the vulnerabilities of unprotected wheels and axle ends. 